A photographic system is composed of several stages, which together function to record and generate a reproduction of an original scene. A pleasing reproduction requires a specific relationship between the luminance of the scene, and the density of the reproduced image. Conventional photographic systems are designed such that the characteristic responses of all stages taken together produce the desired relationship between the original scene recorded and the reproduced image. Each stage in the photographic system is subject to variations in its response characteristics which will alter the sought relationship between the original scene and the reproduced image. In each of the classical forms of photography noted below, the final image is intended to be viewed by the human eye. Thus, the conformance of the viewed image to the recorded scene, absent intended aesthetic departures, is the criterion of photographic success.
In classical black-and-white photography, a photographic element containing a silver halide emulsion layer coated on a transparent film support, commonly referred to as a photographic film, is imagewise exposed to light. This produces a latent image within the emulsion layer. The film is then photographically processed to transform the latent image into a silver image that is a negative image of the subject photographed. The resulting processed photographic film, commonly referred to as a negative, is placed between a uniform exposure light source and a second photographic element, commonly referred to as a photographic paper, containing a silver halide emulsion layer coated on a white paper support. Exposure of the emulsion layer of the photographic paper through the negative produces a latent image in the photographic paper that is a positive image of the subject originally photographed. Photographic processing of the photographic paper produces a positive silver image. The image bearing photographic paper is commonly referred to as a print.
In classical color photography the photographic film contains three superimposed silver halide emulsion layer units, one for forming a latent image corresponding to blue (B) light (i.e., Blue) exposure, one for forming a latent image corresponding to green (G) exposure and one for forming a latent image corresponding to red (R) exposure. During photographic processing, complementary subtractive primary dye images are formed--that is, yellow, magenta and cyan dye images are formed in the blue, green and red recording emulsion layers, respectively. This produces negative dye images (i.e., blue, green and red subject features appear yellow, magenta and cyan, respectively). Exposure of color paper through the color negative followed by photographic processing produces a positive color print.
In one common variation of classical color photography reversal processing is undertaken to produce a positive dye image in the color photographic film (commonly referred to as a slide), the image typically being viewed by projection. In another common variation, referred to as color image transfer or instant photography, image dyes are transferred to a receiver for viewing. When positive photographic film images are formed, second generation slides or prints are made by exposing a color photographic material through the color film and reversal processing to form a positive reproduction of the original scene.
The response characteristics of a photographic system are carefully selected to produce a final image that gives a pleasing reproduction of the recorded scene. The ability of the photographic system to deliver the desired final reproduction is dependent on the photographic response of each step in the imaging chain conforming to its intended response. The response characteristics of the photographic system are intentionally chosen to be different depending on the means of viewing the final image. For example, the response characteristics of a photographic system designed to produce a print intended for viewing by reflected light are different than those for a system which produces a transparency image intended for viewing by projection in a darkened surround. Furthermore, the response characteristics of a photographic system may be intentionally altered to produce a final image which is an aesthetic departure from a faithful reproduction of the original scene.
The characteristics of an imaging system can be altered by selecting photographic materials which differ in their response characteristics or by altering the conditions used to photographically process the photographic medium. The ability to alter the contrast of the photographic image produced is limited by the choice of photographic print materials and processes available. In black and white imaging systems it is typical to alter the contrast of the final print made from a negative by selecting photographic papers of different contrasts. Available color photographic papers span a very limited range of contrasts, restricting the photographer's ability to alter the contrast of the final image, with conventional photographic systems.
The color balance of the final print produced by a color photographic imaging system can be altered by adjusting the color of the light source used when exposing the color negative onto the color paper. However, altering the color of the printing light source produces a constant color change throughout the image, equivalent to altering the density of the individual color records of the negative by adding or subtracting a constant amount of density. When differences in contrast exist between the color records of a color negative, the amounts of density change required to produce an image of high quality is dependent on the image densities. Altering the color of the printing light source is not capable of accomplishing the required correction. The inability to correct for differences in contrast between color records results in degradation of the final image produced.
The response of the photographic film is subject to many sources of variation. Sources of variation include manufacturing, length of time and conditions of storage of the photographic film, and photographic processing. Modern film manufacturing methods minimize variations in the freshly manufactured photographic film's ability to record latent images upon exposure to light from an original scene. However, the ability of a photographic film to form a latent image, and the stability of the latent image formed are both subject to variation dependent on the amount of storage time and the conditions of storage. Raw stock keeping (RSK) of a photographic film relates to variations in the photographic film response characteristics dependent on the storage time and conditions prior to exposure of the photographic film to light from the original scene. Latent image keeping (LIK) of a photographic film relates to the stability of the latent image formed during exposure, prior to its photographic processing. Both aspects of photographic film storage stability are subject to significant variations due to the large range of potential storage times and storage conditions routinely encountered in practice. Photographic film response characteristics are also strongly dependent on the conditions of photographic processing. Variations in photographic processing which affect the conversion of latent image to imagewise density arise from variations in the chemical composition, agitation, and temperature of the processing solutions.
All of the effects discussed above, as well as other effects not specifically mentioned, alter the amount of density formed for different levels of exposure. The relationship between the amount of density formed and the input exposure level can be characterized by the rate of density increase with increasing exposure (photographic gamma, .gamma.), the amount of exposure required to achieve a given density (photographic speed), and the amount of density formed in the absence of exposure (D.sub.min). As previously discussed, the photographic response characteristics of each photographic material used in forming the final reproduction of the original scene is critical in determining the quality of the final image. Variations in photographic film response propagate through the photographic system influencing the contrast and color quality of the reproduced image. Process sensitivity is a dominant factor responsible for the variability of photographic response affecting the quality of the reproduced image.
When the processed photographic film is used as an intermediate in the formation of a final image, such as a print, variations in the D.sub.min and photographic speed of the photographic film can be corrected during conventional printing by adjusting the printing times and the color of the printer light source. In practice, adjustments in the printing conditions are automated by use of a "scene balance algorithm" (SBA). The SBA relies on the measurement of one or more densities of the processed photographic film and an empirically determined relationship between the measured densities and the corrections to be applied to the printing conditions which are most likely to yield an acceptable print.
Original versions of the SBA were based on the observation that the average of all colors contained in a typical recorded scene is a gray of medium lightness. A single density measurement of the entire area of the processed photographic film was made and the printing times and printer light source color were adjusted to make the average color and density of the image match this medium gray. This type of SBA can successfully correct hue shifts arising from the scene illuminant (e.g. daylight, tungsten or fluorescent lighting), and sources of variation in photographic film response which uniformly change the color of the scene (e.g. photographic speed and Dmin), only when the average scene color is a medium gray, as is assumed. When the average density of a photographic film image deviates from an average gray due to the distribution of colors contained within the original scene, adjusting the printing conditions to produce a medium gray will result in unacceptable reproductions of the original scene (termed subject failure). For example, a scene containing large areas of water and sky will have an average density that is bluish and should not be printed to average a medium gray. In other examples, scenes containing large areas of snow would be printed too dark and sunset scenes would be printed too light. To improve the performance of printing algorithms in cases where the original scene does not average to a medium gray, the amount of color correction toward medium gray made in the printing operation is adjusted based on the hue of the average density of the photographic film image. For example, if the average photographic film density deviates from gray in the direction of yellow-orange, the source of the deviation is likely to be tungsten illumination and complete correction will usually be successful. However, if the deviation from medium gray is in the direction of cyan, it is likely that there are large areas of sky or water in the original scene and the printing conditions should not be adjusted to produce a medium gray. For each image, the degree of correction towards neutral is determined by the mapping of the amount of adjustment performed based upon the average hue of the image, referred to as the subject failure suppression boundary.
Another method of determining the amount of correction required in the printing operation is to scan all of the images on a strip of photographically processed film and evaluate the deviations from medium gray of all recorded images. Based on assumptions about the relationships between recorded images, adjustments in the amount of correction toward medium gray of the printing conditions are made. When averaged over an entire sequence of images, hue and density changes due to processing will be constant, but those arising from scene composition will tend to average to medium gray.
More sophisticated algorithms attempt to identify the subject recorded on the photographic film in order to discriminate between hue shifts that require color correction and those that do not. Subject recognition requires multiple density measurements of each photographic film image to determine the location, area, and color of objects in the recorded scene. Recognition of scene content can then be used to determine the amount of correction toward medium gray required when printing the photographic film image.
In all cases, SBA's are used to improve the quality of the reproduced images, without the cost associated with custom printing each image recorded on the photographic film. Furthermore, the operation of every SBA is dependent on an evaluation of the hue of the image contained in the processed photographic film. Unexpected deviations in the response characteristics of the processed photographic film from any of the sources discussed above will affect the ability of a SBA to correctly adjust the printing conditions, degrading the quality of the final image reproduction. In some cases, the parameters of the SBA are adjusted to compensate for variations in the photographic response due to variations in photographic processing conditions characteristic of a single photofinishing operation. This adjustment will typically be incorrect for any single strip of photographic film since the SBA adjustments are determined for the average response of many photographic film strips.
The performance of an automated photographic printer can be further improved by objectively characterizing the photographic response of each individual strip of photographic film. Characterizing the photographic response of each strip of photographic film eliminates the need to adjust the operation of the SBA based on the average properties of all photographic films being handled. Variations in photographic response due to both storage conditions and deviations in photographic processing conditions can be objectively corrected if the photographic response for the photographic film in use is characterized.
R. Davis, U.S. Pat. No. 3,718,074, describes a method of placing reference patches on the photographic film in the camera at the time of exposure of each scene. T. Terashita et al, U.S. Pat. No. 4,577,961, describe a similar method in which the reference patches are applied to the photographic film immediately prior to photographic processing. In both disclosures the densities of the reference patches are measured following photographic processing and the information obtained is used to adjust the operation of the SBA and improve the selection of printing conditions for a conventional optical printing device.
Both of the methods described above are capable of detecting deviations in the response characteristics of the photographic film due to variations in manufacturing, storage, and photographic processing. Recognition of variations in photographic film response is used to improve the performance of SBA's and to more reliably adjust the printing conditions to compensate for variations in image hue due to constant changes in density throughout the photographic film image. Constant changes in image density occur when the Dmin or speed of a photographic film is altered. However, in addition to changes in D.sub.min and speed, variations in film manufacturing, storage, or photographic processing affect film gamma. Variations in film gamma result in density changes which depend on film density. Adjustment of the average density of a printed photographic image, as detailed in the methods described above, is not capable of correcting for alterations in photographic film gamma.
With the emergence of computer data processing, interest has developed in extracting the information contained in an imagewise exposed photographic film strip instead of proceeding directly to a viewable image. It is now common practice to extract the information contained in both black-and-white, and color images, by scanning. The most common approach to scanning a black-and-white negative is to record point-by-point or line-by-line the transmission of a scanning light beam, relying on developed silver to modulate the beam. In color photography blue, green and red scanning beams are modulated by the yellow, magenta and cyan image dyes. In a variant color scanning approach the blue, green and red scanning beams are combined into a single white scanning beam which is modulated by the image dyes, and is read through red, green and blue filters to create three separate records. The records produced by image dye modulation can then be read into any convenient memory medium (e.g., an optical disk) where the information is free of the classical restraints of photographic embodiments. Systematic manipulation (e.g., image reversal, hue alteration, etc.) of the image information that would be cumbersome or impossible to achieve in a controlled and reversible manner in a photographic element is readily achieved. The stored information can be used to modulate light exposures necessary to recreate the image as a photographic negative, slide, or print. Alternatively, the image can be viewed as a video display or printed by a variety of techniques beyond the bounds of classical photography--e.g., xerography, ink jet printing, dye diffusion printing, etc.
I. Shishido et al, U.S. Pat. No. 5,060,061, describe a means for correcting the effect of variations in photographic film response characteristics by measuring the density of an unexposed region of the photographic film (D.sub.min). The photographic image is also scanned to give a digital representation of the image densities. Comparison of the measured D.sub.min with the predetermined D.sub.min characteristics of the film type are used to select a predetermined look-up-table (LUT), which is used to modify a digital representation of the image densities, in order to adjust the tone of the image. This approach is limited in that alterations in the tone scale are made based on the characteristics of photographic film D.sub.min only. Variations in photographic speed and gamma of a photographic film can occur without concomitant changes in the D.sub.min. Using the approach of Shishido, deviations in photographic speed and gamma would go undetected and uncorrected in the printing operation. Furthermore, the range of variations which can be corrected is limited to those that have been predetermined. Variations not anticipated will remain uncorrected.